Normally, the metering servovalve comprises a control chamber having a calibrated, pressurized fuel inlet hole. The control chamber is axially delimited by an end wall of the control rod on one side, and by the wall of the chamber on the other, fitted with an outlet or discharge hole. This outlet hole has a calibrated section and is opened/closed by a shutter to vary the pressure in the control chamber with a predetermined gradient. In particular, the shutter is axially movable under the action of an actuator and the axial thrust of a spring.
Injectors with a balanced-type metering servovalve have already been proposed, in which the shutter is subjected to substantially null axial pressure effects in the closed position, for which both the spring preloading and the actuator force can be reduced. In a known injector with balanced metering servovalve, the body of the valve is coupled with another body comprising an axial guide for the actuator anchor, through an intermediate element carrying an outlet hole with calibrated section, which communicates with a discharge passage carried by said other body. The discharge passage comprises an axial segment and a radial segment that exits through a lateral surface of the guide. In particular, the shutter is formed by a sleeve integral with the anchor and engaging in a fluid-tight manner with the axial guide, so as to obtain large fuel passage sections, without shutter rebound phenomena at the end of opening and closing travel.
This servovalve, although being satisfactory from the viewpoint of balancing pressure on the shutter, has the drawback of requiring three different parts to delimit the control chamber and to guide the anchor. Variations in the opening/closing behaviour of the injection nozzle with respect to that planned can be provoked due to the various couplings of these three parts and the flow conditions inside the injector at high fuel pressures.
An injector has also been proposed in which the valve body is in one piece with a shutter guide stem and carries an outlet passage comprising an axial segment and a radial segment. The latter has an accurately calibrated section and is opened and closed by the shutter, for which the servovalve is still of the “balanced” type.
This injector has a drawback due to the fact that the axial segment of outlet passage increases the volume of the control chamber. In order to achieve acceptable reactivity from the servovalve, it is necessary to reduce the diameter of the axial segment. Since the axial segment always has a very long length compared to the diameter, the drill bit needed to make it tends to flex, with high probability of breaking before arriving at the hole of the radial segment, which is why making it is difficult.
Furthermore, as it is necessary that the diameter of this axial segment is as small as possible, it follows that during the manufacture of the valve body, solid particles, such as machining chips for example, can remain trapped inside the blind part of the channel's axial segment. These solid particles, by having dimensions similar to those of the radial calibrated restriction, can even block it, endangering correct operation of the injector. Even a washing operation, with a liquid under high pressure for example, could be insufficient to remove these solid particles.
Since the calibrated section segment of the channel or restriction is radial, it must run onto a cylindrical surface and must match with the axial segment on the inside. Manufacturing of the valve body is therefore difficult and generates inaccuracies and a high reject percentage. In any case, due to the change in flow direction close to the calibrated section segment, disturbances are created in the fuel flow in output, which reduces reactivity.
Finally, due to the high pressure gradient that becomes established in correspondence to the calibrated restriction when the shutter is opened, vapour is formed immediately downstream of the same calibrated restriction. As this calibrated restriction is positioned close to the sealing surface of the shutter on the valve body, cavitation phenomena can arise that damage the sealing seat. In any case, the absence of fuel in the liquid phase in the zone of cavitation results in contact between the shutter and its seat without any form of damping. Both phenomena cause erosion and enormously shorten the life of the servovalve.